1. Field of the Invention
The present invention relates to a photoelectric conversion device and a driving method thereof. The present invention particularly relates to a photoelectric conversion device which outputs a digital signal, and a driving method thereof Further, the present invention relates to an electronic device provided with the photoelectric conversion device.
2. Description of the Related Art
A number of photoelectric conversion devices used for detecting an electromagnetic wave are known, and for example, photoelectric conversion devices having sensitivity to light from the ultraviolet range to the infrared range are collectively referred to as optical sensors. Among them, an optical sensor having sensitivity in a visible light range with a wavelength of 400 nm to 700 nm is particularly referred to as a visible optical sensor, and a large number of visible optical sensors are used for devices which need illuminance adjustment on/off control, or the like depending on human living environment.
In some display devices, ambient brightness of the display device is detected to adjust display luminance. This is because the visibility is improved and unnecessary electric power of the display device can be reduced by detecting ambient brightness by an optical sensor to obtain appropriate display luminance. Examples of display devices which have an optical sensor for adjusting luminance are mobile phones and computers with a display portion. In addition, as well as ambient brightness of the display device, luminance of a backlight of a display device, particularly, a liquid crystal display device is detected by an optical sensor to adjust luminance of a display screen.
In an optical sensor which is a photoelectric conversion device, a photoelectric conversion element such as a photodiode is used in a portion for sensing light, and light intensity (also referred to as illuminance) can be detected based on the amount of current which flows into the photoelectric conversion element. Patent Document 1 discloses an electric charge accumulation optical sensor having the following structure. Electric charge which is accumulated in a condenser (a capacitor) with current flowing from a photodiode in accordance with the amount of incident light is discharged by a constant current circuit (a constant current source), so that a changing potential is detected by a comparator. The time taken for change of the potential which is detected by the comparator is output as a digital signal by using a counter circuit and a latch circuit.
[Reference]
[Patent Document 1] Japanese Published Patent Application No. H6-313840